The ionized Ca++ concentration ((Ca++)) of the cytosol, an important parameter in the regulation of carbohydrate metabolism in liver, is itself regulated through the combined actions of Ca++ translocases of cellular membranes. These Ca++ transport processes have been studied induvidually and under initial-rate conditions, so there is a considerable gap in our knowledge of (Ca++) regulation under intracellular-type conditions where the combined activities of multiple Ca++ translocases achieve a dynamic balance among Ca++ fluxes. This project will measure the ability of isolated fractions of liver mitochondria and/or endoplasmic reticulum (microsomes) suspended in a cytosol-like medium to regulate the latter's (Ca++) at/near the steady state balance of Ca++ transport. Observed constant values of "Ca++", representing the "set point" of mitochondiral (Ca++) buffering, will then be examined for dependence on the extent of prior mitochondrial Ca++ loading, which affects intramitochondrial (Ca++) and thus the rate of Ca++ efflux, and on changes in mitochondrial reduced/oxidized pyridine nucleotide and extramitochondrial phosphate, adenosine tri-, di- and monophosphate and (Mg++), all of which vary with the rate of cellular energy demand and fuel "substrate" utilization.